Summary:
The Kisladag deposit is classified as a gold-only porphyry deposit due to its exceptionally low Cu%/Au ppm ratio (˜ 0.03; Baker et al., 2016). Significant analogues include the Maricunga porphyry deposits (9.8 Moz Au) in Chile and La Colosa (33.2 Moz Au) in Columbia. The low Cu/Au ratio may in part be related to the shallow level of emplacement (< 1 km) and volcanic setting, but also reflects the post-collisional extensional setting of the Miocene in western Turkey (Baker et al., 2016). Nonetheless, the deposit shares many characteristics with typical porphyry systems including: (i) multi-stage porphyry intrusions; (ii) a zoned alteration system that contain a high temperature potassic core, an outer white mica-tourmaline zone (analogous to phyllic alteration in typical porphyry deposits) and late, high level advanced argillic alteration; and (iii) porphyry style stockwork veins.
The Kisladag deposit is hosted by a suite of subvolcanic monzonite porphyry intrusions that are subdivided into Intrusions #1, #2, #2A, and #3. Intrusion #1 is the oldest, and generally best mineralized phase. It forms the core of the system and is cut by the younger porphyritic intrusions. It is an E-W oriented elongate elliptical body in map view (~1,300 m x ~500 m), and in the subsurface has a sill-like form intruding along the contact of the basement and volcanic package. At depth the main body extends beyond the current limit of drilling (~1,000 m). Contacts between Intrusion #1 and the surrounding volcanic rocks are generally obscured by alteration. Contacts with younger intrusions, particularly Intrusion #3, are better preserved. Intrusion #1 has a K-feldspardominant groundmass with plagioclase phenocrysts (up to 30% of the rock by volume), occurring as tabular crystals ranging in size from 0.1 – 5 mm. Biotite is the second most abundant phenocryst phase (up to 10% of the rock) whereas blocky megacrystic K-feldspar phenocrysts, up to 1 cm, are a characteristic of this unit, but are low in abundance compared to plagioclase and biotite phenocrysts (< 2%). Quartz phenocrysts are rare, and are generally rounded or embayed where present. Intrusion #1 lacks amphibole or pyroxene phenocrysts and primary magnetite.
Intrusion #2 occurs as a WNW-oriented elongate body at depth that splits into two apophyses that form semi-circular stocks (both approximately 150-200 m in diameter) at shallower levels where they cut Intrusion #1. Both apophyses are in contact with and cut by Intrusion #3. The rock is a fine- to medium-grained porphyry with abundant (20-30%) plagioclase phenocrysts up to 2 mm in length in a dominantly K-feldspar groundmass. Rare primary biotite and amphibole phenocrysts occur but the unit lacks quartz phenocrysts.
Intrusion #2A occurs in the southeast corner of the pit and is characterized by a very intense clay (kaolinite-smectite-pyrite) alteration throughout that forms a distinct textural and rheological argillic altered sub-domain termed the friable domain. Intrusion #2A forms a circular stock (250-300 m across) that tapers at depth. It intrudes the margin of Intrusion #1 but contact relationships with
Intrusion #3 are not observed. It is a fine- to medium-grained porphyritic rock, but the intense pervasive clay alteration obscures the primary mineral assemblage. Intrusion #3 is the youngest large intrusive body. It forms a semi-circular stock near the center of Intrusion #1, west of the central Intrusion #2 stock and at depth to the west extends into a WNWelongated, subvertical dike-like body. Intrusion #3 is a fine-grained porphyritic unit with 20-30% plagioclase phenocrysts up to 4 mm in length, and lesser quartz and biotite phenocrysts (both < 5%). Amphibole phenocrysts (5-10%) are more abundant than in the other intrusions but are commonly altered to chlorite. This intrusion is magnetic due to the presence of very fine-grained disseminated primary magnetite in the groundmass.
The oldest stage of alteration is a potassic assemblage characterized by the presence of secondary red-brown biotite and abundant pale pink-buff to nearly white K-feldspar. The biotite alteration is most intense in Intrusion #1 where it is associated with the highest gold grades. There are restricted occurrences of secondary magnetite associated with the potassic alteration, occurring as veinlets and as extremely fine-grained crystals intergrown with secondary biotite. A sodic-calcic sub-domain comprising actinolite-albite-magnetite alteration occurs locally within but overprints the potassic alteration and may form a deeper mineralized sodic-calcic core.
The tourmaline alteration is most intense immediately surrounding the potassic zone, however, the associated white mica alteration is more widespread and is particularly abundant on the west side of the deposit and spatially overlaps advanced argillic alteration. Within the intrusions tourmaline commonly occurs as envelopes around quartz ± pyrite veinlets, grading into black quartz-tourmaline matrix-supported hydrothermal breccias containing angular wallrock fragments. Tourmaline alteration of the volcanic rocks is more pervasive, though cross cutting breccia bodies are also present.
Quartz-alunite ± dickite ± pyrophyllite alteration is most abundant as a lithocap and as an alteration halo on the eastern side of the deposit. The advanced argillic alteration generally occurs in thick tabular zones that are localized along either stratigraphic contacts, within favorable volcaniclastic host rocks, or along fracture systems both within intrusions and volcanic rocks. Stratigraphicallycontrolled zones tend to form subhorizontal to gently dipping lithocaps peripheral to the deposit whereas structurally-controlled zones are steep-sided and form linear outcropping ridges. The advanced argillic alteration is typically poorly mineralized, and commonly oxidized with jarosite replacing pyrite.
Argillic alteration is pervasively developed throughout the deposit but is particularly dominant in the western upper levels and throughout much of the surrounding volcanic sequence. Within the deposit the largest zone of intense kaolinite alteration is focused in Intrusion #2A and a second smaller zone is focused in the southwest corner of the pit within Intrusion #1. Smectite, mainly montmorillonite and locally nontronite, commonly overprints biotite in the potassic alteration zone.
Porphyry-style sheeted to stockwork quartz veins occur with the potassic and white mica- tourmaline alteration zones. Veinlets range in width from 0.1 mm to 1 cm, with most being 1-3 mm. Gold occurs as non-refractory, very fine free gold grains (typically less than 10 microns in diameter) that are associated with pyrite, and less commonly other sulfide phases (chalcopyrite and sphalerite), as well as free grains attached to quartz, K-feldspar and albite. Both native gold and electrum (with up to 18 % Ag) have been identified. Other opaque minerals include pyrite, molybdenite and sphalerite, with minor occurrences of tennantite, tetrahedrite, bournonite, chalcopyrite and gold- and bismuthtelluride. The average copper grade of the deposit is low (~ 200 ppm) but increases to typically between 300 and 500 ppm within potassic alteration (Baker et al., 2016).
The gold grains have an average diameter of 3.8 microns with a range in size from 1.1 to 9.6 microns.
Gold in the argillic alteration occurs primarily with pyrite whereas in the WMT alteration the gold grains occur with pyrite and muscovite. In the potassic samples, the majority of gold is hosted in Kfeldspar.